This invention pertains to quantum cascade (QC) lasers, and to apparatus and systems that comprise a QC laser. More particularly, the invention pertains to quantum cascade distributed feedback (QC-DFB) lasers.
QC lasers are unipolar semiconductor lasers that can be designed to emit light over a tunable range of infrared (IR) wavelengths. In particular, such lasers can be designed to emit in the mid-IR spectral region from about 3 xcexcm to about 17 xcexcm. This spectral range is important because, inter alia, absorption lines of many industrial gases and environmental pollutants lie therein. This suggests that QC lasers may have advantageous applications as radiation sources for absorption spectroscopy of such substances in, e.g., sensors for environmental surveys or process control.
A QC laser has a multilayer structure that forms an optical waveguide, including a core region of relatively large effective refractive index disposed between lower and upper cladding regions of relatively small effective refractive index. A cladding region is also referred to as a xe2x80x9cconfinementxe2x80x9d region because the guided optical radiation tends to be confined within the higher-index region, i.e., within the core, whose boundaries are marked by the transition to the lower-index cladding regions.
The core region comprises a multiplicity of nominally identical repeat units, with each repeat unit comprising an active region and a carrier injector region. The active region has a layer structure selected to provide an upper and a lower energy state for carriers, which are typically electrons but may alternatively be holes. The separation between the upper and lower states is such that a carrier transition from the upper to the lower state may result in emission of a photon at an infrared wavelength xcex.
The carrier injector region has a layer structure selected to facilitate the transport of carriers occupying the lower energy state of the active region of a given repeat unit to the active region of the adjacent, downstream repeat unit, such that the transported carriers will occupy the upper energy state of the downstream active region. A carrier thus undergoes successive transitions from an upper to a lower energy state as the carrier moves through the layer structure, meanwhile emitting multiple photons of wavelength xcex. The photon energy, and thus the wavelength xcex, depends on the structural and compositional details of the repeat units.
One desirable property of QC lasers, particularly in applications involving precise spectroscopic measurements, is single-mode operation, as defined by a suppression of side modes by a factor of typical order 1000 (in measurements of the optical power). Early QC lasers were incapable of single-mode emission in continuous wave (cw) operation at cryogenic temperatures (below about 140 K) and in pulsed operation at room temperature. Typical emission spectra of QC-lasers are multiple-mode.
U.S. Pat. No. 5,901,168, which issued on Apr. 27, 1999 to J. N. Baillargeon et al. under the title xe2x80x9cArticle Comprising an Improved QC Laserxe2x80x9d and is commonly assigned herewith, describes an improved QC laser capable of single-mode operation. The single-mode operation was achieved through the use of a grating structure for providing distributed feedback. The grating structure was specifically designed to induce single-mode operation. Because a grating structure was used for distributed feedback, the laser described there belonged to the class referred to as QC-DFB lasers.
Significantly, the benefits of single-mode emission were achieved in U.S. Pat. No. 5,901,168 using a grating structure fabricated on the upper surface of the device, and not a buried grating structure formed adjacent the core layer (although the cited patent also reported on results obtained using such a buried structure). The use of an overlying, and not a buried, grating structure is advantageous because it substantially simplifies the fabrication process. However, because of greater separation from the core layer, an overlying grating structure has generally been believed to offer weaker coupling of the optical mode to the grating than a buried grating structure, and consequently to have a weaker effect. In particular, coupling via a modulation by a surface grating of the effective refractive index of the modes was believed to be inefficient. In fact, early QC-DFB lasers fabricated with surface gratings did not operate in grating-induced single-mode emission when operated in cw. The latter requires a strong coupling between the guided mode and the grating.
There remains a need for QC-DFB lasers having still higher output powers and, in particular, operable with single-mode cw output. However, mode instability tends to be aggravated as temperatures and power levels increase. Strongly mode-coupled structures are needed to counter such spectral instability. Thus, one recognized problem in the art has been to provide mode stabilization sufficient to extend the range of single-mode operation with respect to optical output power and possibly operating temperature, particularly for cw conditions, while continuing to enjoy the convenience afforded to the fabrication process by using overlying, rather than buried, grating structures.
We have devised an overlying grating structure that achieves relatively strong coupling of the guided mode to the grating, and is thus highly effective in inducing a QC-DFB laser for single-mode operation even under cw operating conditions. Using such a grating structure, we have demonstrated continuously tunable, pulsed, single-mode operation at room temperature, and the first grating-induced continuously tunable, cw, single-mode operation at and above liquid nitrogen temperature, i.e., at and above 77K.
Our invention is embodied in an article (e.g., an absorption spectroscopy system) that comprises a QC-DFB laser. The QC-DFB laser comprises first and second cladding regions and a core region disposed between them. The core region comprises a multiplicity of multilayer semiconductor repeat units selected for lasing at a wavelength xcex. The QC-DFB laser further comprises a grating structure for providing distributed feedback to facilitate single-mode laser operation. The grating structure overlies the core and is separated therefrom by at least a portion of the upper cladding region. The grating structure is overlain by an upper metallic electrode.
At least part of the grating structure is formed as a series of grooves (also referred to herein as xe2x80x9cvalleysxe2x80x9d) etched in a semiconductor layer, denominated the plasmon-enhanced confinement layer (PECL), which is disposed adjacent and in contact with the upper metallic electrode. The refractive index of the PECL at the lasing wavelength xcex is typically significantly lower than the refractive index of any of the cladding layers that underlie it. A guided waveguide mode of the laser will experience an effective refractive index at the midpoint positions of the grooves that is different from the effective refractive index at the midpoint positions between the grooves.
The thickness, refractive index (as adjusted through doping), and etched depth of the PECL are chosen such that the effective refractive index changes between the intra-groove midpoint positions and the inter-groove midpoint positions by at least 2xc3x9710xe2x88x923 and possibly by as much as 1xc3x9710xe2x88x922 or even more, although a currently preferred value is about 5xc3x9710xe2x88x923.
The properties of the PECL are chosen such that near the intra-groove midpoint positions, where PECL material is substantially or even entirely removed, the laser mode travelling in the waveguide can couple efficiently to the surface-plasmon, which exists at the interface between the upper cladding layers and the overlying electrode. This has the effect of xe2x80x9cpullingxe2x80x9d the mode toward the surface and thereby changing its overlap with the various layers constituting the waveguide. Near the inter-groove mid-point positions, where the PECL is intact, the coupling to the surface-plasmon is at least partially suppressed, effectively xe2x80x9cpushingxe2x80x9d the mode away from the surface. This xe2x80x9cpush-pullxe2x80x9d action (periodic with the grating period) is responsible for a relatively strong modulation of the refractive index. Secondarily, it also induces a modulation of the modal gain via a modulation of the confinement factor, defined as the overlap of the guided mode with the waveguide core, i.e., with the multiplicity of active regions and injectors. (It should be noted that the spatially modulated coupling to the inherently lossy surface-plasmon furthermore induces a loss modulation.)
A xe2x80x9cpushxe2x80x9d or xe2x80x9cpullxe2x80x9d of the laser mode refers to a relative decrease or increase, respectively, in amplitude of the electromagnetic field of the laser radiation near the feature of interest, i.e., the PECL or grating. The amount of xe2x80x9cpush-pullxe2x80x9d can be quantified by the change in confinement factor, and is preferably designed to be approximately 10%-20% (as absolute percentages of the waveguide confinement), but can be made even higher if desired.